(1) Field of the Invention
The present invention relates to an image forming apparatus, and in particular relates to an image forming apparatus such as an electrostatic copier, laser printer, facsimile machine or the like that includes a developing device using a dual-component developer containing a toner and a magnetic carrier and forms images based on electrophotography using toner as well as relating to a toner supply method for the image forming apparatus.
(2) Description of the Prior Art
Conventionally, image forming apparatuses based on electrophotography such as copiers, printers, facsimile machines and the like have been known. The image forming apparatus based on electrophotography is constructed so as to form an image by forming an electrostatic latent image on the surface of a photoreceptor, e.g., photoreceptor drum, supplying toner to the photoreceptor drum from a developing device to develop the electrostatic latent image, transferring the toner image formed on photoreceptor drum by development to a sheet of paper etc., and fixing the toner image onto the sheet by means of a fixing device.
Recently, in the image forming apparatuses supporting full-color and high-quality images, a dual-component developer (which will be referred to hereinbelow simply as “developer”), which presents excellent charge performance stability, is often used.
This developer consists of a toner and a carrier, which are agitated in the developing device and frictionally rubbed with each other to thereby produce appropriately electrified toner.
The electrified toner in the developing device is supplied to a dual-component developer supporting member, e.g., the surface of a developing roller. The toner thus supplied to the developing roller is moved by electrostatic attraction to the electrostatic latent image formed on the photoreceptor drum. Hereby, a toner image based on the electrostatic latent image is formed on the photoreceptor drum.
Further, recently, image forming apparatuses are demanded to be made compact and operate at high speeds, hence it is necessary to electrify the developer quickly and sufficiently and also convey the developer quickly and smoothly.
To deal with this, in order to disperse supplied toner promptly into the developer and provide the toner with an appropriate amount of charge, a circulating type developing device has been adopted in image forming apparatuses.
This circulating type developing device includes: a developer conveying passage in which the developer is circulatively conveyed; a screw auger (developer conveying member) for conveying the developer while agitating in the developer conveying passage; a toner supply port for leading toner from a toner container into the developer conveying passage; and a toner concentration detecting sensor for detecting the toner concentration of the developer. In this arrangement, when the toner concentration of the developer becomes lower than a predetermined level, toner supply is directed to the toner cartridge so that toner is supplied to the developer conveying passage and the supplied toner is conveyed whilst being agitated (see Patent Document 1: Japanese Patent Application Laid-open 2006-106194).
In the aforementioned circulating type developing device using the dual-component developer, if toner to be supplied from the toner cartridge to the developing device is used up, the toner concentration in the developer gradually degreases, so that carrier transfer phenomena (carrier adherence) to the photoreceptor drum occur more frequently. As a result, it is necessary to perform toner empty decision.
Toner empty decision is to decide (detect) the state of toner empty when, for example, the toner concentration of the developer detected by the toner supply sensor does not increase after toner supply was directed to the toner cartridge.
However, if the toner concentration detecting sensor is located away from the toner supply port through which toner is supplied, toner empty decision is delayed because the fall in toner concentration detected by the toner concentration detecting sensor is slow, posing the problem that carrier adhesion occurs more frequently.
On the other hand, when the amount of supplied toner is detected by a sensor disposed near the toner supply port, it is necessary to use the average (moving average) of the latest multiple sampled values in order to improve detection accuracy. Correcting toner supply control based on this average value makes it possible to stabilize the amount of supplied toner.
However, when the toner cartridge is replaced by another one, the moving average is not pertinent, so that there is a fear of excessive toner supply or insufficiency of toner supply occurring during the period after replacement of the toner cartridge until the moving average converges to the normal value after multiple number of samplings.